High Performance Phototransistor Based on 0D-CsPbBr₃/2D-MoS₂ Heterostructure with Gate Tunable Photo-Response

Abstract

Monolayer MoS₂ has been widely researched in high performance phototransistors for its high carrier mobility and strong photoelectric conversion ability. However, some defects in MoS₂, such as vacancies or impurities, provide more possibilities for carrier recombination; thus, restricting the formation of photocurrents and resulting in decreased responsiveness. Herein, all-inorganic CsPbBr₃ perovskite quantum dots (QDs) with high photoelectric conversion efficiency and light absorption coefficients are introduced to enhance the responsivity of a 2D MoS₂ phototransistor. The CsPbBr₃/MoS₂ heterostructure has a type II energy band, and it has a high responsivity of ~1790 A/W and enhanced detectivity of ~2.4 × 10¹¹ Jones. Additionally, the heterostructure CsPbBr₃/MoS₂ enables the synergistic effect mechanism of photoconduction and photogating effects with the gate tunable photo-response, which could also contribute to an improved performance of the MoS₂ phototransistor. This work provides new strategies for performance phototransistors and is expected to play an important role in many fields, such as optical communication, environmental monitoring and biomedical imaging, and promote the development and application of related technologies.

1. Introduction

A heterostructure built using two-dimensional materials with strong light absorption ability can be used when designing high performance photoelectronics, such as phototransistors and photodiodes [1,2,3]. Two of the most popular approaches are building photodiodes using two different 2D materials and phototransistors based on 2D semiconductors as channels [4,5].

Transition metal dichalcogenides (TMDCs)-MoS₂ have shown great potential in fabricating photodetectors, including phototransistors and photodiodes, in the last decade [6,7,8,9,10]. M. Park et al. constructed vertical WSe₂/MoS₂ heterojunctions to enhance the responsivity of near-infrared light. The responsivity reached 0.3 A/W thanks to the highly effective generation, separation and transportation of photocarriers [11]. In 2021, Y. Mao et al. built lateral MoS₂ Schottky photodetectors and the maximum responsivity reached 1.9 A/W (wavelength~1310 nm). A gold electrode absorbs most of the incident light through the transparent MoS₂ layers, and a photocurrent gain is generated in the MoS₂ layers [12]. In 2024, L. Wang et al. used tellurium because the (Te) microwire and polyvinyl alcohol (PVA) combine to form a functional flexible substrate to improve the detection performance of an MoS₂ device in near-infrared range. A responsivity of 0.28 mA/W and a detectivity of similar to 1.41 × 10¹⁰ Jones were realized by using a 808 nm laser [13].

However, most MoS₂ photoelectronic devices are mainly targeted at visible and near-infrared light absorption or detection caused by narrow bandgap of MoS₂ (1.2–1.8 eV), which limit their further application in photoelectronics [14]. It has been found that 0D quantum dots (QDs) showed promising absorption ability in a wide range spectrum and they could be easily integrated with two-dimensional materials to form a van der waals heterostructure for designing high performance optoelectronics [15]. Konstantatos et al. used PbS quantum dots as a photosensitive layer to build a PbS/graphene photodetector, which achieved an increased responsivity from 10⁻² A/W to 10⁷ A/W [16]. Next, the researchers set out to combine different quantum dots with TMDCs to demonstrate the role of quantum dots in improving photoelectric performance. They could modulate the charge density in TMDC-based optoelectronic devices when illuminated with light [17,18,19]. In 2018, Sangyeon Pak et al. prepared an MoS₂/PbS QDs heterostructure phototransistor and the detectivity reached 1 × 10¹¹ Jones because of the enhanced generation of photoexcited carriers in PbS QDs [20]. In addition, all-inorganic halogen perovskites with the molecular formula CsPbX₃ (X = Cl, Br, I) showed more competitive prospects in new optoelectronic application due to their adjustable band gap, expected absorption efficiency and easy fabrication process [21,22,23,24,25]. And it was a promising strategy to integrate CsPbX₃ with a high carrier transport layer and a high light absorption layer for constructing optoelectronic devices with improved photon–electron conversion efficiency.

In this work, we designed and fabricated a heterostructure phototransistor by using MoS₂ nanosheet and CsPbBr₃ QDs. Here, monolayer MoS₂ nanosheets synthesized by chemical vapor deposition (CVD) method were used as the conductive channel of the phototransistor. CsPbBr₃ QDs were spin-coated on MoS₂ nanosheets as the light absorption layer. The device exhibited an enhanced responsivity (1790 A/W) and a higher detectivity (2.4 × 10¹¹ Jones) in ultraviolet light compared to a bare MoS₂ phototransistor. Here, the generated photoelectron–hole pairs in CsPbBr₃ would separate and the electrons would rapidly inject into MoS₂, which reduced the charge recombination and accelerated the transport of the carriers [26,27]. The increased photocurrent and improved photo-response resulted from the synergistic effect of both photoconduction and photogating in the CsPbBr₃/MoS₂ heterostructure phototransistor. This work laid the foundation for the development of high performance optoelectronics based on 2D materials and 0D perovskite.

2. Materials and Methods

2.1. Fabrication of CsPbBr₃ QDs

For CsPbBr₃ fabrication, CsBr (0.1 mmol, Beijing Yinokai Technology, China), PbBr₂ (0.1 mmol, Shanghai McLean Biochemical Technology, China) and 10 mL DMF (Beijing Yinokai Technology, China) solution were first mixed at room temperature, then oleic acid (0.25 mL, Tianjin Xinsi Biochemical Technology, China) and oleamine (0.13 mL, Tianjin Xinsi Biochemical Technology, China) were added to stabilize the precursor solution. Stirring continued until all the precursor powders were completely dissolved into a suspension. Finally, 1 mL precursor suspension and 10 mL xylene (Tianjin Kemi Ou chemical reagent, China) were mixed and we obtained CsPbBr₃ QDs solution. It could be seen that with natural light excitation, the quantum dot solution emitted yellow light and when using ultraviolet light as excitation source, the quantum dot solution emitted green light (Figure 1a).

2.2. Fabrication of Monolayer MoS₂ Nanosheets

MoS₂ nanosheets were fabricated on a SiO₂/Si substrate (Suzhou silicon electronic Technology, China) using the NaCl assisted CVD method, with MoO₃ (Alfa Aesar, China) and S (Alfa Aesar, China) powder as precursors and NaCl as the catalyst. Here, the mass ratio of MoO₃/NaCl was 1/2 (wt%), and Ar was used as carrier gas with a flow of 20 sccm. The deposition process was achieved in a double temperature zone tube furnace, the reaction temperature was set to ~750 °C and remained for 30–35 min, ensuring the full reaction of all precursors.

2.3. Fabrication and Measurements of CsPbBr₃/MoS₂ Phototransistor

First, the MoS₂ nanosheets were transferred onto a new SiO₂/Si substrate (0.05 Ω·cm) and used as conductive channel; e-beam lithography was implemented, followed by evaporating Ti/Au (10/80 nm) as the source/drain electrode. After that, CsPbBr₃ QDs solution was fabricated on MoS₂ to form a CsPbBr₃/MoS₂ heterostructure (Figure 1b). All electrical properties were measured using a semiconductor parameter analyzer (Agilent, B1500, Santa Clara, CA, USA) and a 405 nm wavelength laser (Figure 1c).

3. Results

3.1. Characterizations of CsPbBr₃/ MoS₂ Heterostructure

The chemical stability of each layer in the CsPbBr₃/MoS₂ heterostructure was first proved by XPS test. Typical peaks of Mo 3d, S 2p, Cs 3d, Pb 4f and Br 3d were all observed in the wide-scan spectrum, as shown in Figure 2a. The characteristic peaks of Mo 3d were located at 233 and 223 eV (Figure 2b), S 2p at 164 and 163 eV (Figure 2c), Cs 3d at 734 and 724 eV (Figure 2d), Pb 4f at 143 and 138 eV (Figure 2e) and Br 3d at 68.5 eV (Figure 2f), respectively. The results were highly consistent with bare MoS₂ and CsPbBr₃ QDs, suggesting the high stability of both CsPbBr₃ and MoS₂ in the heterostructure. Meanwhile, the molecular structural stability of MoS₂, CsPbBr₃ QDs and the CsPbBr₃/MoS₂ heterostructure was evaluated by Raman characterization with an excitation laser of 532 nm (5 mW). Here, ${E }_{2g}^1$ and $A_{1g}$ characteristic peaks of MoS₂ were located at 380.54 cm⁻¹ and 403.73 cm⁻¹, respectively, while the original CsPbBr₃ QDs showed no obvious Raman peaks. In the CsPbBr₃/MoS₂ heterostructure, the Raman peaks were still dominated (Figure 2g) by MoS₂ and there were no obvious shifts compared to bare MoS₂, indicating that MoS₂ had high quality without any damage during the preparation process.

The fabricated MoS₂ had a clean surface and specific triangle topography with thickness of ~0.74 nm (monolayer, Figure S1). The PL spectra of bare monolayer MoS₂ (668 nm), bare CsPbBr₃ QDs (525 nm) and the CsPbBr₃/MoS₂ heterostructure were excited by a 405 nm laser (Figure 2h). As expected, the PL peak of the CsPbBr₃/MoS₂ heterostructure showed excitation peaks both in 525 (CsPbBr₃ QDs) and 668 nm (MoS₂), suggesting independent photo-generated carriers and photon emission in each layers. There was a large drop in PL intensity of CsPbBr₃/MoS₂ compared to bare MoS₂ nanosheets or CsPbBr₃ QDs, which might be caused by the effective carrier transport from CsPbBr₃ to MoS₂; it also proved that CsPbBr₃ as a photosensitive layer could indeed realize the effective separation of carriers. On the other hand, the PL peaks of all layers exhibited a narrow FWHM, suggesting a high crystal quality.

Furthermore, the light absorption capability of MoS₂, CsPbBr₃ QDs and the CsPbBr₃/MoS₂ heterostructure were tested and the corresponding ultraviolet-visible (UV-vis) spectra are shown in Figure 2i. The absorption intensity of the CsPbBr₃/MoS₂ heterostructure was significantly increased compared with bare MoS₂ or CsPbBr₃. In particular, the CsPbBr₃/MoS₂ heterostructure contained the absorption peaks of both MoS₂ and CsPbBr₃, and significantly enhanced at the range of 400~500 nm and 617~661 nm, which might be due to the formation of new electronic states and band structures in the CsPbBr₃/MoS₂ heterostructure. At the same time, the similar absorption capacity of the CsPbBr₃/MoS₂ heterostructure and bare CsPbBr₃ QDs confirmed the effective photosensitive layer role of CsPbBr₃ in this heterostructure.

From Figure 3a, the CsPbBr₃ nanocrystals uniformly distributed on the MoS₂ nanosheet and Cs, Pb, Br and Mo elements could be observed from the energy dispersive spectrum (EDS) mapping in Figure 3b–e, respectively. The specific element ratio according to Figure 3f further verified the contained elements of this CsPbBr₃/MoS₂ var der waals heterostructure.

3.2. Electrical Performance of CsPbBr₃/MoS₂ Phototransistor

The electrical performance of CsPbBr₃/MoS₂ and the bare MoS₂ phototransistor under dark conditions is shown in Figure 4a. The current of the CsPbBr₃/MoS₂ heterostructure phototransistor was increased by nearly 25% compared to bare MoS₂ transistor. This might be because of the spontaneous transfer of electrons from MoS₂ to the CsPbBr₃ layer. Figure 4b shows the transfer characteristic curves of the CsPbBr₃/MoS₂-phototransistor (I_ds − V_gs), which was similar to the bare MoS₂-FET. The almost linear relationship of the out-put characteristic curves in Figure 4c indicated good ohmic contact between the metal electrode and the conducting channel. The device showed an acceptable on/off ratio of ~10³ according to Figure 4d. The carrier mobility was calculated by Formula (1),

$$\mathsf{\mu }={\displaystyle \frac{{dI}_{ds}}{{dV}_{gs}}}\times {\displaystyle \frac{L}{W}}\times {\displaystyle \frac{1}{C_g{V}_{gs}}}$$

(1)

where W/L is the channel width/length, and C_g is the capacitance of the dielectric layer [28]. Here, L/W are 2/5, respectively, and the calculated mobility is 2.32 cm² V⁻¹s⁻¹. The transconductance in Figure 4e increased first and reached the maximum value, as the carrier concentration increased according to the gate voltage. Then it gradually reduced with the increasing gate voltage because of the saturation of the carriers.

3.3. Photoelectrical Performance of CsPbBr₃/MoS₂ Phototransistor

Further, we revealed the photosensitivity of the CsPbBr₃/MoS₂ phototransistor by using a 405 nm laser as the illumination source. The photocurrent (laser power = 0.1 W/cm²) of the device steadily increased, with V_ds increasing from 0.5 to 2.5 V, indicating that the photo-generated carriers mainly contributed to an increase in the channel current (I_ds) in this device under illumination (Figure 5a). Then we studied the device performance using different laser power (0.05~5.6 W/cm², V_ds = 1 V) in Figure 5b. The threshold voltage of the CsPbBr₃/MoS₂ phototransistor gradually moved towards the negative direction as the laser power increased. This might be attributed to the trapping of photo-generated holes at the interface of CsPbBr₃/MoS₂, which electrostatically modulated the carrier. When the laser power differs, the photocurrent is affected (Figure 5c). The channel current illuminated by the external light was higher than that under dark conditions, and it further increased according to the incident light power, which implied that the photocurrent was dominated by the photoconduction effect, in the CsPbBr₃/MoS₂ heterostructure phototransistor in this case.

From the band alignment diagram of the CsPbBr₃/MoS₂ phototransistor under dark and light conditions, it could be seen that under dark conditions (Figure 5d), a depletion layer was formed due to the difference in the Fermi levels of MoS₂ and CsPbBr₃, and only a small number of electrons transferred spontaneously from CsPbBr₃ to MoS₂; thus, the current was weak. After illumination, a large number of photo-generated carriers (electron–hole pairs) would be produced in CsPbBr₃ (Figure 5e), and the internal electric field would drive more electrons transfer from CsPbBr₃ to MoS₂, greatly increasing the photocurrent [29].

The relationship between drain voltage, power density and photocurrent were obtained (Figure 5f). The fabricated heterostructure phototransistor had higher I_ds at V_ds and increasing laser power, suggesting the CsPbBr₃/MoS₂ heterostructure had good light absorption capability and thus, contributed to a higher photo-response. A transient light-response in Figure 5g shows an obvious switching characteristic of the CsPbBr₃/MoS₂ phototransistor under a fixed laser power of 0.1 W/cm² and stable photocurrent with low dark current.

The photocurrent was calculated by Formula (2),

$$I_{ph}=I_{light}-I_{dark}$$

(2)

where I_dark is 1.5 × 10⁻⁵ A (V_gs = 20 V). Typically, we evaluated light absorption (R) of the phototransistor by Formula (3),

$$\mathrm{R}={\displaystyle \frac{I_{ph}}{\mathrm{P}\times \mathrm{S}}}$$

(3)

where P is the incident light power intensity, S is the effective irradiation area of the device and the effective area S is 4 μm² in this work. Responsivity (R) of the CsPbBr₃/MoS₂ phototransistor decreased as the laser power increased from 0.05 to 5.6 W (Figure 5h). Specific detectivity (D*) was calculated according to Formula (4),

$$D*={\displaystyle \frac{\mathrm{R}\sqrt{\mathrm{S}}}{\sqrt{2\mathrm{e}{I}_{dark}}}}$$

(4)

where e is the electron charge and S is the effective irradiation area of the device. The results showed that R and D* reached the maximum value of 1790 A/W and 2.4 × 10¹¹ Jones, respectively, with a 405 nm laser (intensity of 0.05 W/cm²), which were significantly improved compared to bare MoS₂ phototransistor (330 A/W and 4.4 × 10¹⁰ Jones in Figure S2).

The change in photocurrent under different gate voltage (Figure 6a) showed that the photocurrent was closely related to the gate voltage and optical power. Under illumination, the photo-generated electron–hole pairs in CsPbBr₃ were separated and then transferred to MoS₂, forming the photocurrent (I_ph). When the light intensity increased, the I_ph of the CsPbBr₃/MoS₂ phototransistor also increased correspondingly, as the photo-generated carriers increased with the increasing laser power, indicating a photoconduction effect. Moreover, the I_ph of the CsPbBr₃/MoS₂ phototransistor increased when the gate voltage swept from −50 to −10 V (Figure 6b), and then saturated at ~−10 V. Power law fitting was carried out through I_ph~P^α (Figure 6c) and α was the slope. The photocurrent had a linear relationship with the incident light power (i.e., α = 1), indicating the photocurrent was proportional to the incident light power and the photoconduction effect was dominated in the device [30]. When 0 < α < 1, the photocurrent had a nonlinear relation with the incident light power. At this time, with the gate voltage decreasing from −40 to 40 V, the dependence of the photocurrent on the incident power increased nonlinearly, indicating that the photogating effect dominated the generated photocurrent at this time.

The gate-modulated effects in Figure 6d describe the electron–hole pair generated when CsPbBr₃ absorbs photons during illumination. Due to the difference in Fermi levels between CsPbBr₃ and MoS₂, the depletion layer provided a built-in electric field that promoted the transfer of electrons from CsPbBr₃ to the MoS₂ layer and thus, increased the carrier concentration and I_ph (photoconduction). Meanwhile, those holes were limited in the CsPbBr₃ layer and generated a positive gate effect towards the MoS₂ channel. As the gate voltage increased (Figure 6e), more carriers were generated in the MoS₂ channel; thus, contributing to an enhanced photogating effect, as well as increased I_ph, which would saturate as the gate voltage continually increased.

Alternatively, while I_ph/I_dark was obviously regulated by the gate voltage (Figure 6f), it also identified the influence of the photogating effect on I_ph. Both responsivity (R, Figure 6g) and detectivity (D*, Figure 6h) of the CsPbBr₃/MoS₂ phototransistor were improved compared to the bare MoS₂ device, and they were strongly associated with the gate voltage, e.g., R reached its maximum value at ~0 V (V_g) and D* showed a peak value at ~−30 V (V_g). Additionally, the special band gap of MoS₂ and CsPbBr₃ might have promoted the transfer of electrons between the van der waals layers and, we realized, a phototransistor with violet light absorption capability, high responsivity and detectivity (

Table 1. The responsivity and detectivity of various phototransistors based on 2D materials.

Structure	MoS2 Layer	Wavelength (nm)	Responsivity (A/W)	Detectivity (Jones)	Ref.
MoS2	monolayer	640	50	-	[31]
MoS2	multilayer	633	0.12	1010	[32]
MoS2/MoTe2	few layers	637	4.6 × 10−2	1.06 × 107	[33]
MoS2/CH3NH3PbI3	few layers	638	1.1	9 × 1010	[34]
MoS2/2DPI	monolayer	900	390.5	5.10 × 1012	[35]
CsPbBr3/MoS2	monolayer	405	1790	2.39 × 1011	This study

).

Table 1 summarizes the detection bands, responsivity and detectivity of phototransistors with different structures based on the MoS₂ materials reported in recent years. It can be found that the 0D perovskite quantum dots/MoS₂ heterostructure phototransistor has effectively improved the responsivity in this work. And the photocurrent is increased by nearly 20% with ultraviolet light as an excitation source, which provides certain data support for the research of two-dimensional material optoelectronic devices.

4. Discussion

In summary, we demonstrated a high-performance phototransistor by integrating a high carrier transport layer (2D-MoS₂) and an outstanding photo absorption layer (CsPbBr₃ QDs). The fabricated CsPbBr₃/MoS₂ phototransistor had a wide absorption spectrum between 300 and 800 nm. In addition, the decline in PL peaks also demonstrated the effective transfer of charge from CsPbBr₃ to MoS₂. Due to the high absorptivity and effective interfacial charge separation of the CsPbBr₃/MoS₂ heterostructure phototransistor, we obtained an increased responsivity of 1790 A/W and a detectivity of 2.4 × 10¹¹ Jones. The mechanisms of photocurrent generation and regulation were also identified by modulating the laser power and gate voltage. Both photoconduction and photogating effects were present and synergistically contributed to the generation of photocurrent, and photogating effects became dominant with the higher gate voltage. The results showed that the coupling of 2D nanomaterials with the perovskite layer was an ideal choice to realize the next generation of high-performance phototransistor.